Cytotoxic CD8+ T cell (CTL) responses play a critical protective role against HIV virus. A number of studies, however, have indicated that HIV-specific CD8+ T cells may be functionally impaired. Furthermore, we have demonstrated that HIV-specific CD8+ T cells exhibit increased susceptibility to undergo CD95/Fas-mediated apoptosis and HIV-infected macrophages can kill these cells. This apoptosis of HIV-specific CD8+ T cells therefore may impair their ability to function as serial killers. Augmenting the effector function and the survival of HIV-specific CD8+ T cells would greatly enhance the efficiency of these cells to control or clear HIV virus. Interleukin 15 (IL-15) is a pluripotent cytokine that we have shown potently inhibits CD95/Fas-mediated apoptosis of HIV-specific CD8+ T cells and upregulates the anti-apoptotic Bcl-2 and Bcl-xL molecules in these cells. Furthermore IL-15 enhances their cytotoxic activity and IFNgamma production. SIV-specific CD8+ T cells from SIV-infected rhesus macaques also show increased sensitivity to CD95/Fas-mediated apoptosis and IL-15 potently inhibits this apoptosis and upregulates Bcl-2 and Bcl-xL molecules. Based on these observations, we recently performed a short pilot study in chronically SIV-infected cynomolgus macaques to investigate the potentially beneficial effect of IL-15 treatment in vivo. In vivo treatment with IL-15 significantly increased peripheral blood CD8-t- T cell and NK cell numbers by more than 2-fold. This increase was mainly due to proliferation of CD8+ T cells, as proliferating Ki67^ CD8+ T cells increased with treatment. The expanded CD8+ T cells were of the CD45RA- CD62L- and CD45RA+ CD62L- effector memory phenotype. We hypothesize that IL-15 treatment in vivo can enhance cytotoxic CD8+ T cells immunity against SIV and enhance antiviral immunity. We propose to examine the effect of in vivo treatment with recombinant simian IL-15 on primary and chronic SIV infection of rhesus macaques. Synergy with ART treatment will also be evaluated in chronic infection studies. Viral load, immunological changes, apoptosis and Bcl-2/Bcl-xL molecule expression will be evaluated. In particular the effect of in vivo IL-15 treatment on SIV-specific CTL response will be investigated. The studies in this proposal are novel and will provide valuable information on the potential therapeutic value of IL-15. Information yielded from these studies may prove useful for the design of novel therapeutic strategies against HIV infection and other viral infections.